The invention relates generally to defibrillators and, more particularly, to an apparatus for adjusting the energy of a defibrillation pulse.
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient""s chest. While most contemporary external defibrillators have applied monophasic waveforms to patients, biphasic waveforms are now being used more frequently due to research that indicates that a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
The American Heart Association has recommended a range of energy levels for the first three defibrillation pulses applied by an external defibrillator. The recommended energy levels are: 200 joules for a first defibrillation pulse; 200 or 300 joules for a second defibrillation pulse; and 360 joules for a third defibrillation pulse, all within a recommended variance range of no more than plus or minus 15 percent according to standards promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). These high energy defibrillation pulses are required to ensure that a sufficient amount of the defibrillation pulse energy reaches the heart of the patient and is not dissipated in the chest wall of the patient.
High energy defibrillation pulses in these ranges are generally designed for certain types of defibrillation in adults. While most external defibrillators are designed to provide defibrillation pulses at these energy levels, other applications may require lower energy defibrillation pulses. For example, low energy defibrillation pulses may be required when defibrillating babies or small children, or when internal paddles are coupled to the defibrillator for use in surgery to directly defibrillate the heart, or for cardioversion of some arrhythmias in both pediatrics and adults.
With regard to babies and small children, the AHA guidelines call for energy settings of 2 joules per kilogram for neonatal defibrillation and 0.5 joules per kilogram for synchronous cardioversion with an Edmark waveform. Designing an external defibrillator so as to be able to provide these low energy levels that are required for babies, as well as the normal high energy levels that are required for adults, increases the complexity and cost of an external defibrillator. Accordingly, there is a need for a simplified, cost-effective design for an external defibrillator that can provide low energy defibrillation pulses appropriate for children, as well as the normal energy defibrillation pulses for adults.
Another consideration with regard to the energy levels of defibrillation pulses is in regard to varying patient impedance levels. More specifically, when a defibrillating pulse is applied to a patient, the pulse encounters a resistance to the flow of electrical current through the patient. The resistance of a patient""s thorax to the flow of electrical current is called transthoracic impedance (TTI). The magnitude of current flowing through a patient is directly proportional to the magnitude of the voltage difference across the electrodes used to deliver the defibrillation pulse to the patient and inversely proportional to the patient""s TTI.
External defibrillators are likely to encounter patients with a wide range of TTI values. Thus, one challenge that is faced by external defibrillator manufacturers is to design defibrillators that work well over a wide range of patient TTI values. With regard to defibrillators that are designed to apply pulses to adults, while such conventional defibrillators are often specified for and tested with 50 ohm loads, adult patient TTI can vary greatly in a range from 25 to 180 ohms. Average adult patient TTI in a hospital setting is about 80 ohms. Children""s TTIs can also vary over wide ranges.
Defibrillator circuits which generate damped sine and truncated exponential pulses respond differently to variations in transthoracic impedance. Damped sine defibrillator impedance response is passive; that is, the response is determined entirely by the amount of capacitance, inductance, and resistance in the circuit. As impedance increases, defibrillating pulse duration increases and peak current decreases.
Several factors affect the shape of waveforms produced by truncated exponential defibrillators in response to different TTI values. Both the capacitance and resistance of the circuit determine passively how quickly the current drops after its initial peak. The active control of a switch that truncates the discharge determines the duration of each phase of the pulse. By design, pulse duration typically increases with increasing TTI values. This is done to allow additional time for energy delivery before the pulse is truncated.
Prior art defibrillators that are designed for adult defibrillation are calibrated for energy delivery at a single, specified load impedance, typically 50 ohms. However, as noted earlier, the TTI of many adult patients exceeds 50 ohms. As a result, the amount of energy actually delivered to a patient is different than the energy level selected by the operator. With damped sine waveforms, patients with TTI greater than 50 ohms receive higher energy than the energy level selected by the operator. With truncated exponential waveforms having fixed durations, patients with TTI greater than 50 ohms receive less energy than the selected energy level. The peak current delivered to patients also drops as patient TTI increases. Prior art defibrillators using truncated exponential waveforms typically adjust the duration of the waveforms (i.e., increased duration with increased impedance) to compensate for a decrease in energy delivered. However, partly because of a reduction in peak current produced in higher impedance patients, long duration truncated exponential waveforms may be less effective among high impedance patients. See, for example, the article xe2x80x9cTransthoracic Defibrillation of Swine with Monophasic and Biphasic Waveforms,xe2x80x9d Circulation 1995, Vol. 92, p. 1634, in which the authors Gliner et al. acknowledge that, for a biphasic truncated exponential waveform, pulse durations exceeding 20 milliseconds are less effective.
Recognizing that patient TTI values affect the amount of current actually delivered to a patient, the prior art has proposed various techniques designed to compensate for varying patient impedance values. A number of these prior art techniques are discussed in commonly assigned U.S. Pat. No. 5,999,852, to Elabbady et al., which is hereby incorporated by reference. Elabbady et al. also discuss a method by which the patient""s TTI is used to control the amount of energy contained in a defibrillation pulse that is applied to the patient.
Such prior art external defibrillators are typically designed to provide shocks with a waveform having either a fixed pulse width or a fixed tilt or droop. If the pulse width is fixed, then the tilt would vary inversely with patient impedance. Conversely, if the tilt is fixed, then the pulse width would vary according to patient impedance. Thus, these conventional external defibrillators require additional circuitry that enables the defibrillator to adjust the shock waveform so as to achieve the selected amount of energy in the given waveform, based on the patient impedance. This additional circuitry tends to increase the complexity and cost of the external defibrillator. Accordingly, there is a need for a simplified, cost-effective design for an external defibrillator that can provide a waveform with a relatively fixed pulse width and tilt over an expected range of patient impedances.
The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to an energy adjusting circuit for a defibrillator that produces a low energy defibrillation waveform with a fixed pulse width and fixed tilt.
In accordance with the present invention, an energy adjusting circuit is provided for a defibrillator. The energy adjusting circuit adjusts the defibrillation pulse energy that would otherwise be applied to the patient. The energy adjusting circuit can be part of the defibrillator itself, or part of an adapter coupled to the output ports of a conventional defibrillator. In an adapter embodiment, the adapter can optionally have paddles configured for use on small patients such as babies and small children, so as to avoid confusion with the regular defibrillation paddles that are otherwise connected to the defibrillator.
In accordance with another aspect of the invention, the energy adjusting circuit comprises a divider circuit. The divider circuit is used to dissipate a predetermined portion of the shock energy so that a predetermined low energy pulse is delivered to the patient. In one particular embodiment, two resistors are connected to form an energy divider, with the paddles being connected across one of the resistors. The resistance ratio of the two resistors is predetermined so that a predetermined percentage of the defibrillation pulse energy is provided to the patient. In particular, the resistance values are predetermined so that in conjunction with the patient impedance, the external defibrillator scales the full energy shock in the predetermined ratio so as to deliver a shock with the desired energy level.
In accordance with yet another aspect of the invention, in an embodiment where two resistors are connected to form an energy divider, an isolation network may be connected in series with the second resistor. The paddles may then be connected across the series connection of the isolation network and second resistor. The isolation network helps allow ECG signals to be monitored via the therapy electrodes/paddles. As another embodiment, the first resistor may be divided into two resistors, one of each being placed in series with each of the two connections back to the defibrillation energy and control circuitry. The dividing of the first resistor into two separate resistors helps mitigate the effect of the series resistance of the first resistor.
In accordance with still another aspect of the invention, the external defibrillator can be configured to recognize that the adapter is present and to scale the displayed energy level settings. For example, the external defibrillator may have energy settings ranging from 2 joules to 360 joules and the adapter may have a 10:1 energy reduction ratio. With this feature, the external defibrillator would recognize the presence of the adapter and would display or in some manner indicate that the energy settings range from 0.2 joules to 36 joules.
In accordance with yet another aspect of the invention, the divider circuit reduces the effect of patient impedance on the equivalent impedance of the xe2x80x9cnetworkxe2x80x9d formed by the divider circuit and the patient impedance. Thus, different patient impedances will not significantly affect the tilt of the waveform. By optimizing this effect, the external defibrillator, in effect, delivers shocks having a fixed pulse width and a fixed tilt. As a result, the need for adjusting the waveform based on patient impedance is significantly reduced or even eliminated.
In accordance with a further aspect of the invention, the external defibrillator can be configured to deliver a waveform with an optimal pulse width and tilt. For example, the defibrillator can be configured to generate a waveform with the pulse width and tilt designed to maximize successful treatment, or reduce the size of the storage capacitor or some other parameter.
In accordance with yet another aspect of the invention, the external defibrillator can be configured to have a single charge level (i.e., the energy storage capacitor is always recharged to the same predetermined level) but provide variable energy level shocks through the use of one or more energy adjusting circuits.